The present invention relates generally to recombinant lead acid batteries. More particularly, the invention relates to a rechargeable recombinant lead acid battery having positive and negative plates coated with active material and separated by a compressible separator material containing an electrolyte. A casing provides structural support for the plates and encases the battery's components in such a way that the casing maintains a substantially evenly distributed compressive force on the separator material substantially greater than the pressures normally found within a typical recombinant lead acid rechargeable battery.
The most common and probably the best known battery chemistry is lead acid. One advantage of lead acid batteries is that they have very repeatable power delivery characteristics and may be recharged and overcharged repeatedly with minimal damage to the cells. Additionally, the power curve is consistent enough that the charge remaining in a cell at any given time can be relatively accurately predicted by merely measuring the cell's potential. Thus, a user can be easily warned well in advance of a loss of power. A significant improvement to the traditional lead acid battery design is the recombinant battery. The recombinant lead acid battery differs from its predecessors in that substantially all of the electrolyte is absorbed within the separator between adjacent plates and/or an active paste applied directly to the plates. Gases evolved during operation or charging are not normally vented into the atmosphere, but rather are induced to recombine within the battery. With such an arrangement, no free acid is available, which allows the battery to be sealed and maintenance free. The elimination of free acid also provides a safer battery design.
An important technical requirement of lead acid batteries is that the spacing between plates must be maintained at a constant distance. As is well known in the art, if the plates of a lead acid battery are not sufficiently constrained, the plates will expand and the battery degrades relatively quickly. Accordingly, the casing must be sufficiently strong to prevent separation of the plates under the influence of the considerable forces that can act on the plates during a charge/discharge cycle. Two areas of prior art recombinant batteries are particularly bulky and/or heavy. The first is the electrodes which are typically fabricated from lead grids and the second is the casing structure which is typically heavily reinforced. A significant improvement in the traditional recombinant lead acid battery design was disclosed in applicant's U.S. Pat. No. 4,996,128 which described a recombinant lead acid battery having battery plates that are formed from metallic foils. As illustrated in FIG. 1, the battery described in the '128 patent and generally indicated by reference numeral 20 has matching upper and lower external casing members 22 and 24 and a terminal arrangement 30. The internal structure includes spaced apart positive and negative plates 42 and 44 which are pasted with active materials 45 and 47 respectively. A separator 49 electrically isolates the plates. A multiplicity of pins 50 extend through the plates and separator to internally support the casing in order to maintain a constant spacing between the plates during operation and recharging. The pins 50 are formed from complementary pin portions 52 and 54 which are integrally formed with the upper and lower casing members 22 and 24 respectively. This structure provides a lightweight battery with high energy and packaging densities.
Although the arrangement disclosed in the '128 patent is quite advantageous compared to other conventional lead acid batteries, it, along with other recombinant lead acid batteries, is susceptible to a phenomenon referred to as Premature Capacity Loss (PCL). This phenomenon refers to a rapid reduction in the dischargeability of fully charged positive plate material much earlier in the life of the battery than expected. Several theories have been put forth to explain PCL. However, no precise understanding of the mechanism causing PCL as yet been discovered and agreed upon.
PCL is not to be confused with the cycle life and the normal, expected degradation of a rechargeable lead acid battery. Cycle life typically refers to the expected number of times a rechargeable battery may be discharged and recharged before the normal or expected degradation of the active materials within the battery adversely limit the batteries performance. To the contrary, PCL occurs much earlier in the expected life of the battery and is not typified by the expected degradation of the active materials associated with normal cycle life degradation. Accordingly, there is a need for an improved battery design that reduces the chances of the PCL phenomenon occurring within the battery.